1. Field of the Invention
This invention relates to information handling and control systems, and to methods of testing the condition of electrical loads in such systems.
2. Description of Related Art
An example of such a system in the form of an automotive vehicle electrical system is known from the article "Multiplexing for the automotive industry" by W. R. Betts in GEC REVIEW, Vol. 2, No. 1, 1986 at pages 32 to 36.
FIG. 1 of the accompanying drawings is an overall schematic diagram corresponding to the Figure shown on page 34 of the GEC REVIEW article.
In FIG. 1 there is shown a star wired system with an intelligent, central, master unit 10 and a number of non-intelligent, local, slave units 20. Power from a battery +V is supplied to the master unit 10 and via fuses 30 to the slave units 20. A low current signalling link 40 is connected from the master unit 10 to each slave unit 20. Electrical switches, sensors, and high current loads (not shown in FIG. 1) are connected to the slave units 20 and their interaction is controlled from the master unit 10 by means of time slot assignment multiplex.
In the GEC REVIEW article it is mentioned that each signalling bus (link) has a clock line and a data (signal) line. The clock line carries bursts of clock pulses, usually eight or sixteen pulses per burst which define the time slots for the data line. For sixteen time slots each slave unit thus provides sixteen interfaces (channels), and during each time slot a single digital data bit or analogue level is transferred between that slave and the master on the data line, for example a command from the master to the slave to operate a load, such as a lamp or a motor, or an indication of the state of a load from the slave to the master. In a discussion of testability on page 36 of this article it is mentioned that by monitoring the current drawn from the battery the state of each load can be individually checked. This implies that a dedicated separate one of the slave channels is required for testing each load and this implication is confirmed by a block diagram of the slave unit on page 35 of this article which shows one group of channels connected to loads via an "outputs" block and these loads connected via a "diagnostics" block to a separate group of channels.
For a system having a given number of channels provided at each slave unit as described above, then for each slave unit to which it is required to connect one or more loads the provision of a separate test channel for each load is disadvantageous in limiting the number of loads and possibly also switches and sensors which can be connected to the slave unit.